There are a number of conventional processes for packaging integrated circuits. One approach which is commonly referred to as “flip chip” packaging generally contemplates forming solder bump contacts (or other suitable contacts) directly on an integrated circuit die on a wafer. After the contacts are formed, the dice are singulated by sawing or cutting the wafer along the scribe lines. The individual die can then be “flipped” and attached to a substrate such as a printed circuit board. That is, the solder bumps on the die are aligned and mounted onto matching contacts on the substrate. The solder bumps are then reflowed to electro-mechanically connect the die to the substrate.
When a flip chip die is mounted to the substrate, an air gap typically remains between the die and substrate. This gap is commonly filled with material that is flowed into the gap in liquid form and is then solidified. This material is generally a mixture of a epoxy resin and small silica spheres and is often called underfill. A dispenser or nozzle is typically used to dispense the liquid underfill material at one edge of the die. The material then flows into the narrow gap due to capillary action and spreads across the flip chip die until finally the entire area of the gap between the die and substrate is filled. The underfill material is then cured.
Since the silicon of the flip chip package and the substrate have different coefficients of thermal expansion, the solder joints may fail during normal operation. The solidified or cured underfill material helps maintain the integrity of the solder joints, which in turn, helps to reduce failure of the joints in the field.
As a general rule with wafer-level chip scale packages, it is desirable that sufficient underfill material be present so that a fillet is formed around the die when it is mounted onto a substrate, such as a printed circuit board. During this process, the elevated temperatures causes the partially cured underfill material to flow and to fully cure. Fillets of underfill material are formed as a result around the periphery of the wafer-level chip scale packages. Generally speaking, the more underfill material present on the periphery of the die, the larger and more robust the fillet. It is therefore desirable to have additional underfill material at these locations. For more details on the benefits of fillets, see “Effects of Underfill Fillet Configuration on Flip Chip Packaging Reliability”, by H. Nguyen et al., the International Electronics Manufacturing Technology Symposium, SEMICON West, July, 2002.
There are problems associated with aforementioned type of underfill process. For example, the operation of applying underfill must be repeated for each flip chip mounted onto a substrate. Repeating such an operation many times during manufacturing significantly increases costs.
An apparatus and method for enhancing the formation of fillets around the periphery of assembled wafer-level packages when mounted onto substrates is therefore needed.